ok, ended up doing this last night:
the low was -10 with light winds
used a full and a short thermarest
bivy
-20 sierra design 800 fill bag
base layer, fleece layer, insulated pants, down jacket with the zip off sleeves over the feet, base layer gloves and down mitts, face shield over the nose
kicked out a trench in about 7 inches of snow
was out for about 7 hours, was fully warm and estimate this would be true for another 5 degrees and some more wind
certainly this system would be reasonably comfortable to - 25 and keep one alive overnight to - 35
no big issues with condensation / frost as I try to expel my breath outside the bivy
some cold spots on the feet when they were pushing outward/ compressing the insulation - felt no cold spots from the ground